Right, so using the 1804 and 1806 designations in the Dakotas, the westbound ('outbound') uses the north/east banks of rivers and the eastbound return (inbound') uses the south/west banks of rivers. Montana and Idaho are a bit different as its where they split up on the way back, and because they don't follow a river over the mountains.
'OUTBOUND'
Ohio River
in.lctrl
Missouri River
mo.lctrl
ia.lctrl,sd.lctrl,nd.lctrl
Colombia River
wa.lctrl
INBOUND
Colombia River
or.lctrl
or.lctrlher
Missouri River
nd.lctrlwes,sd.lctrlwes,ne.lctrl
ks.lctrl,mo.lctrlsou
Ohio River
ky.lctrl (not yet finished making wpt)
COULD GO EITHER WAY
id.lctrl - links to wa.lctrl (which is 'outbound') and mt.lctrlbla (which is mostly inbound-only in terms of route)
SPURS/CONNECTORS
nd.lctrlkee
nd.lctrlnew
nd.lctrlpic
nd.lctrlwas
nd.lctrlbis
nd.lctrlsbi
sd.lctrlmob
sd.lctrlpie
mo.lctrlwes
MONTANA - these routes may need reforming (and are really terribly signed, though I've not checked it that thoroughly yet)
mt.lctrlmis (Missouri River Downstream) - at least the bit east of 'Forks of the Yellow' is unsigned. Runs on northside, so outbound.
mt.lctrlfor (Forks of the Yellowstone) - outbound, unsigned, is really part of 'Missouri River Downstream' (with the bit on US2 north of it some sort of bypass/truck route leg)
mt.lctrl (Missouri Headwaters) - eastern bit south of the river so inbound (Lewis), western bit outbound, far western bit inbound (Clark) as well as outbound. Some signs at Dillon
mt.lctrlyel (Yellowstone River) - inbound (Clark), some signs in Billings area
mt.lctrlbla (Blackfeet) - from Bitterroot Mountains section eastward is inbound (Lewis), westward to Idaho border is both
mt.lctrlbit (Bitterroot Mountains) - outbound and inbound (Clark)